In recent years, a need exists for handling RF signals in a plurality of frequency bands with a single terminal in a portable telephone and a wireless local area network (LAN) system. Conventionally, as a high-frequency (RF) amplifier for use in such an application, amplifiers 11, 12, . . . , 1n and impedance conversion circuits 21, 22, . . . , 2n have been generally provided in parallel one by one exclusively for each of respective frequency bands f1, f2, . . . , fn having RF signals, for example, as illustrated in FIG. 1.
In the configuration illustrated in FIG. 1, since amplifiers 11, 12, . . . , 1n are provided in parallel exclusively for each of the frequency bands, the number of parts for dedicated amplifiers 11, 12, . . . , 1n have increased together with an increase in RF signals in frequency bands different from one another, causing a problem that the size of the area needed for mounting the parts has increased as well as the cost of the entire amplifier.
For solving the problem as mentioned above, there is a technique which amplifies RF signals in two frequency bands with a single amplifier, as disclosed, for example, in JP-A-11-97946. Describing the technique disclosed in this official publication with the same reference numerals given to parts common to FIG. 1, the output of amplifier 1 between impedance matching circuit 2 and impedance conversion circuits 21, 22 is switched by switch circuit 5 to amplify RF signals in two frequency bands as illustrated in FIG. 2, or is switched by first bandpass filter 61 and second bandpass filter 62 to amplify RF signals in two frequency bands as illustrated in FIG. 3.